Cement is a material with adhesive and cohesive properties that make it capable of bonding mineral fragments into a compact whole. The cement most commonly used in civil engineering and building is Portland cement. Concrete is produced by intimately mixing cement, water, fine aggregate (sand), and coarse aggregate (gravel). The mixture is then placed in forms, compacted thoroughly, and allowed to harden.
Concrete is strong under compression, but relatively low in strength under tension. When a structural member such as a beam is made of concrete, it is under both compressive stress at the top of the beam and tension at the bottom of the beam. Thus, a concrete beam would tend to fail by being cracking and pulling apart at the bottom, where the stress is tensile. The same is true of concrete roads, or any other application where tensile forces will be applied to concrete.
This can be overcome by placing reinforcement where it is necessary for structural members to resist tensile forces. The result is called "reinforced concrete." The reinforcement is typically in the form of steel bars (usually called "reinforcing bars" or simply "rebars") or welded wire fabric (in the case of flat areas such as roads, floors, or other concrete slabs).
In a concrete beam, the steel rebar is placed in the lower part of the beam, so that the tensile forces are countered by the reinforcement. The steel reinforcement is bonded to the surrounding concrete so that stress is transferred between the two materials.
In a further development the steel is stretched before the development of bond between it and the surrounding concrete. When the force that produces the stretch is released, the concrete becomes precompressed in the part of the structural member that is normally the tensile zone under load. The application of loads when the structure is in service reduces the precompression, but generally tensile cracking is avoided. Such concrete is known as "prestressed concrete".
There are a number of drawbacks to steel reinforcement in concrete construction, at least in certain applications.
The durability of concrete structures, in corrosive environments such as bridge beams and decks and parking garage floors, is determined by the life of the reinforcement steel. In the snowy Northeast, the salt applied to roads in the winter leaches into the concrete and causes the reinforcing bars to rust away. This has resulted in the need for massive road reconstruction in recent years, as the road and bridge slabs poured in the construction of the Interstate Highway System in the 1960's begin to crack and fail all over the Northeast. The safety implications for reinforced concrete used in beams and decks in road bridges and parking structures are frightening.
In other applications, the steel in conventional reinforced concrete may be a problem. Magnetically levitated (MagLev) trains depend on strong magnetic fields and linear induction motors which may be disrupted by metallic reinforcing in supporting beams.
There have been several attempts in the past at replacing steel reinforcing bars with bars which are partly or entirely made of non-metallic materials. These have, in general, resulted in round reinforcements made of continuous fibers such as carbon, aramid or glass in resin, which resemble the traditional steel rebars. Because of the failure of the concrete to adhere properly to the plastic reinforcement, the rebars are commonly wrapped in spirals of additional plastic material to provide something for the concrete to grip. This has not been entirely successful, for several reasons: the helical wrapping or rings intended to provide grip tend to slip along the length of the rebar; the lack of a secure method of anchoring the reinforcement; and, if the external helical wrapping is stressed so that indentations occur, these indentations create sharp kinks in the longitudinal fibers initiating failure; in a pultrusion process, as the diameter increases, the strength of the resulting bar is reduced due to non-uniform curing across the bar.
L'Esperance, et al., U.S. Pat. No. 4,620,401, is typical of these earlier non-metallic reinforcing bars. The bar is formed by a process called "continuous pultrusion", whereby the fibers are drawn through a bath of resin, wrapped with the spiral fibers for mechanical adhesion to the concrete, sprayed with more resin, and cut to length. Pultrusion poses problems for reinforcing bars because of non-uniform strength because of non-uniform wetting of the fibers and curing of the resin. As the diameter increases the strength per unit area tends to decrease. L'Esperance has outside "embossments", corresponding to the transverse ribs of the present invention. Without the cover plies of the present invention, these "embossments" will tend to slide along the bar when put under stress.
Goldfein, U.S. Pat. No. 2,921,463, is a glass fiber reinforcement for concrete which has no ribs or other means to increase adhesion with the concrete. The glass fibers are bound with a resin which is still wet when the fibers are placed in the concrete, or in a separate operation, cement is applied to the wet resin to create a primer to which the concrete can bond.
Abbott, U.S. Pat. No. 3,167,882, is a method of prestressing concrete. A rod made of two parts of different materials, bonded with a bonding agent which can be destroyed by heat. The core part, made an electrically conductive material such as steel or cast iron, is prestressed in compression. Layers of a material having a high tensile stress, such as a suitable steel alloy or fiberglass, are bonded to the sides of the core while the core is under compression. After the bonding is complete, the compression in the core is released, and the compressive force becomes a tension in the side layers. The rod is set into the concrete and after the concrete cures an electrical current is passed through the core, heating it and releasing the bond between the core and the side layers.